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Full text of "Text Book Of Mechanical Engineering"

Appendix VL

1027

The direction of rotation being shewn by the arrow, it is evident
that the tool must be gradually pushed farther into the cut for
each tooth, and then suddenly withdrawn to begin upon the next
tooth; while, at the same time, a longitudinal traverse of either a
parallel or spiral form is to be maintained as required. We shall
now examine the motions of the lathe to see how the aforesaid
operations may be performed.

The leading screw E provides the longitudinal feed or traverse,
being driven by change wheels at D, and having the usual rocker
lever H to reverse the motion when required (see Figs. 135-6,
p. 145). The spiral shaft c, as it is called, is driven by spur
wheels at B, a change of speed being obtained by the sliding
spindle G, which puts one or other of the wheels at F in mesh at
will; and a rocker lever is also provided at K for reversal. Follow-
ing shaft c rightwards, it is seen to drive a shaft M through the
medium of differential bevels, so that if wheel r be fixed, the
pinions L roll round and rotate M at half the speed of c. Change
wheels N are then so arranged as to turn shaft P, called the cam
shaft, at the speed required to cause a prescribed number of teeth
to be formed upon the cutter blank ; in other words, the rotations
of P are always an aliquot number of parts of those of M, and at
present also of c, though the latter will only hold when r is
stationary and parallel-toothed mills are being cut. The cam
shaft P is so called because it rotates the backing-ofF cam u
through the bevel wheels x and a vertical shaft, giving the required
transverse motion to the upper slide x of the saddle w.

The longitudinal feed is made spiral by means of a curious
connection between the leading screw and the spiral shaft. The
screw E, supplied with change wheels R at its right-hand end,
drives a worm gear Q through the mitre wheels s, thus causing a
very slow rotation of the wheel r. The shaft M no longer turns at
the exact half-speed of M, nor do P'S rotations divide exactly into
those of c, though the old relationship is retained between p & M.
The effect is to cause the reciprocation of the tool slide x, as given
by the cam u, to be very slightly retarded, so that at every rota-
tion of the work the tool enters at a somewhat later or lower
position. Simultaneously a leftward feed has been given by the
leading screw, and thus a left-hand spiral path is traced upon the